This invention relates to printers, and more particularly to techniques for improving print quality and for extending printhead life in ink-jet printers.
Ink-jet printers operate by sweeping a printhead with one or more ink-jet nozzles above a print medium and applying a precise quantity of ink from specified nozzles as they pass over specified pixel locations on the print medium. One type of ink-jet nozzle utilizes a small resistor to produce heat within an associated ink chamber. To fire a nozzle, a voltage is applied to the resistor. The resulting heat causes ink within the chamber to quickly expand, thereby forcing one or more droplets from the associated nozzle. Resistors are controlled individually for each nozzle to produce a desired pixel pattern as the printhead passes over the print medium.
To achieve higher pixel resolutions, printheads have been designed with large numbers of nozzles. This has created the potential for printhead overheating. Each nozzle firing produces residual heat. If too many nozzles are fired within a short period of time, the printhead can reach undesirably high temperatures. Such temperatures can damage and shorten the life of a printhead. Furthermore, widely varying printhead temperatures during printing can change the size of droplets ejected from the nozzles. This has a detrimental effect on print quality.
Printhead overheating is often the result of a high xe2x80x9cdot densityxe2x80x9d during a single swath of the printhead. When making a swath, the printhead passes over a known number of available pixels, some of which will receive ink and others of which will not receive ink. The pixels that receive ink are referred to as dots. The xe2x80x9cdot densityxe2x80x9d is the percentage of pixels in a swath that receives ink and thereby become dots. When printing many types of images, such as text images, dot densities are relatively low and do not cause overheating. More dense images such as photographic images, however, require a much higher dot density and create the distinct potential for overheating.
Another problem caused by printing high-density images is that there might be insufficient ink in the nozzle area of the printhead for printing the next swath. Over time, firing a nozzle when it has an insufficient supply of ink will destroy the nozzle.
Generally, it is known to deal with both these problems by pausing the printhead. Where excessive printhead temperature is a concern a pause is utilized to allow the printhead to cool. Similarly, a pause is used to allow additional ink to flow into the nozzle area of the printhead.
The above referenced application, SWATH DENSITY CONTROL TO IMPROVE PRINT QUALITY AND EXTEND LIFE IN INK-JET PRINTER, describes techniques which address these problems, including disabling nozzles in the printhead, and providing reduced-height swaths to reduce throughput. This application provides additional techniques for addressing these problems.
A method is described for controlling average printing density over time in an inkjet printer having a printhead with a plurality of nozzles, the printhead mounted in a scanning carriage for producing a print swath across a print medium. The method includes:
moving the carriage to the printhead repeatedly across a print medium in individual swaths;
firing individual nozzles repeatedly during each printhead swath to apply an ink pattern to the print medium;
reducing the carriage velocity during a particular swath.
The carriage velocity reduction can occur as a result of one of several occurrences. For example, the step of reducing the carriage velocity can be performed in response to high print densities that are predicted to raise the printhead temperature to unacceptably high levels.
In accordance with another aspect of the invention, an inkjet printer that applies an ink pattern to a print medium is described, and includes control logic, a printhead, and a carriage for mounting the printhead. The carriage is responsive to the control logic to pass the printhead repeatedly across the print medium in individual swaths, the printhead having individual nozzles that are fired repeatedly during each printhead swath to apply an ink pattern to the print medium. The control logic determines a swath dot density prior to each swath. If the swath density of an upcoming swath is greater than a maximum permissible swath density, the control logic acts to reduce the carriage velocity during the upcoming swath to produce a swath with reduced print density.